1. Field of the Invention
The present invention relates to a method for operating an at least three-phase electric machine, used as a drive assembly in a motor vehicle, and a control unit for an inverter.
2. Description of the Related Art
To drive hybrid or electric vehicles, electric machines are used, as a rule, in the form of polyphase machines, which are operated in connection with rectifiers, that are frequently also designated as inverters. The electric machines, in this context, are operated optionally in motor or generator operation. In motor operation, the electric machine generates a driving torque which, when used in a hybrid vehicle, supports an internal combustion engine in an acceleration phase, for example. In generator operation, the electric machine generates electrical energy, which is stored in an energy store, such as a battery or a Super-Cab. The operating type and the power of the electric machine are set using a control unit, frequently designated as machine controller, via the inverter.
Known inverters include a series of switches, with which the individual phases (U,V,W) of the electric machine are optionally switched to a high potential, the so-called intermediate-circuit voltage, or to a low reference potential, particularly ground. The switches are actuated by an external control unit which, as a function of the driver's command (accelerating or braking) calculates a setpoint operating point for the electric machine. The inverter is connected to the control unit and receives from it the appropriate operating data and control commands
The operation of the electric machine and the inverter are monitored for safety reasons, and in response to errors, these are restricted or adjusted.
Thus, for instance, a method known from published German patent application document DE 10 2006 003 254 A1, for switching off an electric machine using pulse-controlled inverters in the case of interference, in which undesired side effects during the switching off of the electric machine are minimized and to maximize the regular machine operation, in that the electric machine is first switched to a disconnect operation in which all the switches of the pulse-controlled inverter are open, and subsequently switched to a short-circuit mode, in which the switches connected to high potential are open and the switches connected to the low potential (ground) are closed.
However, in order to maintain the availability of a vehicle even in the error case, it is known for a hybrid vehicle, from published German patent application document DE 10 2008 000 904 A1, that one may ascertain at least one operating parameter value which reproduces an operating parameter of the electric machine, determine an error if at least one of the operating parameter values does not correspond to an operating parameter normal state and to restrict the operation of the electric machine as a driving drive assembly at least partially if a detected error is present. For this, when a detected error is present, the electric machine is operated at least temporarily as a starter, independently from operating the electric machine as a travel drive assembly.